This invention is concerned with germanium-containing glasses demonstrating very good transmission properties in the infrared portion of the radiation spectrum and are of a relatively low density, along with the process for their manufacture. Glasses exhibiting good transmission of infrared radiation are widely used in both civilian and military applications.
Germanium-containing glasses which were designed for transmitting infrared radiation are known to the art. For example:
U.S. Pat. No. 3,119,703 discloses alkali metal-free glasses in the base composition system BaO-TiO.sub.2 -GeO.sub.2. The glasses are asserted to transmit infrared radiation at wavelengths out to about six microns, to exhibit high deformation temperatures, to display good resistance to chemical attack, and to consist essentially, expressed in mole percent on the oxide basis, of
______________________________________ GeO.sub.2 35-45 BaO 18-22 TiO.sub.2 10-30 La.sub.2 O.sub.3 7 ZrO.sub.2 3-5 ThO.sub.2 0-3 Ta.sub.2 O.sub.5 0-1 PbO 0-2 WO.sub.3 0-2 ZnO 0-10 BaF.sub.2 0-4. ______________________________________
U.S. Pat. No. 3,531,305 describes glasses in the base composition system GeO.sub.2 -Al.sub.2 O.sub.3 -CaO exhibiting high transmittance at wavelengths between 5-6 microns. The glasses consist essentially, expressed in mole percent on the oxide basis, of
______________________________________ GeO.sub.2 33-42.5 Al.sub.2 O.sub.3 20-30 CaO 30-40 TiO.sub.2 and/or La.sub.2 O.sub.3 and/or CdO and/or ZnO and/or BaO and/or SrO and/or MgO 0-7. ______________________________________
The patent also explains the importance of reducing the water content in the glass and teaches the preparation of glasses having low water contents via the addition of a chemically reactive, chlorine-containing agent to the batch, e.g., CaCl.sub.2, and melting the batch in the presence of a dry atmosphere flowing directly over the glass melt. Thus, the "residual water" (OH.sup.- hydroxyl in content) must be minimized since it absorbs very strongly at a wavelength of about 2.9 microns.
U.S. Pat. No. 3,745,032 reveals glasses statedly transmitting about 80% of the infrared radiation at wavelengths out to 5.5 microns. The glasses consist essentially, expressed in mole percent on the oxide basis, of
______________________________________ GeO.sub.2 40-80 CaO 1-15 ZnO 2-10 ZrO.sub.2 1-10 MgO 5-16 K.sub.2 O and/or Na.sub.2 O 1-10 Al.sub.2 O.sub.3 0-15. ______________________________________
U.S. Pat. No. 3,769,047 discusses glasses manifesting excellent transmittance of infrared radiation out to 5.5 microns and which evidence good abrasion resistance and low liquidus temperatures, the latter feature greatly improving the melting and forming characteristics of the glass. The glasses consist essentially, expressed in weight percent on the oxide basis, of
______________________________________ GeO.sub.2 25-50 Al.sub.2 O.sub.3 1.5-25 La.sub.2 O.sub.3 15-40 Nb.sub.2 O.sub.5 and/or Ta.sub.2 O.sub.5 and/or SrO and/or BaO and/or PbO and/or ZnO 2.5-45. ______________________________________
U.S. Pat. No. 3,911,275 discloses infrared transmitting glasses of high abrasion resistance and having a relatively low coefficient of thermal expansion. The glasses consist essentially, expressed in weight percent on the oxide basis, of
______________________________________ GeO.sub.2 25-50 La.sub.2 O.sub.3 10-50 Ta.sub.2 O.sub.5 5-50 ZnO 2-20 Nb.sub.2 O.sub.5 0-40 Al.sub.2 O.sub.3 0-5 ZrO.sub.2 0-5 PbO 0-20. ______________________________________
However, the glasses described in those patents have generally had a relatively high density, viz., greater than about 3.31 g/cm.sup.3. For some applications, e.g., the fabrication of windows for missiles, there has been a need for light weight glasses exhibiting a high degree of infrared transmission, especially at wavelengths between 3.5-5 microns. Low density, infrared transmitting glasses have been marketed commercially but such have generally demonstrated a transmission of no more than about 50% at a wavelength of 4.4 microns in a thickness of 2 mm.